BoilerFaultCodesThe 4 most common faults
01
H9 High Engineer
Your boiler has shut down because the exhaust gases have reached a temperature that is too high, which is a safety feature to prevent damage to the system.
Full guide for H902
L9 High Engineer
Your boiler has sensed that the exhaust gases are getting too hot and has shut down as a safety precaution to prevent internal damage.
Full guide for L903
H n Medium Engineer
Your boiler is displaying a service notification or a specific lockout state that requires a professional to check the internal sequence and settings.
Full guide for H n04
HE Low DIY-safe
Your boiler is displaying an 'H' and 'E' sequence to indicate its current operating status or a diagnostic mode, often triggered by a system reset or power-up.
What to try first
Observe the boiler for 5-10 minutes to see if it clears the cycle automaticallyCheck that your external heating controls or room thermostats are calling for heatPerform a simple reset by turning the control knob to 'Reset' and back to 'On'
on the Evo HE 12
Main Heat ExchangerFlue ThermistorBurner Insulation PanelHeat exchangerFlue thermistorMain PCB
Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.
Call a Gas Safe engineer if…
- You can smell gas or see signs of a leak
- The Evo shows an Emergency or High severity code
- The boiler keeps locking out after repeated resets
- You've tried the DIY checks and the fault hasn't cleared
- There's visible water leaking from the boiler
- The flame is yellow or orange instead of blue